Signal conversion arrangements are known which respond to the reception of a digital input signal and generate an output signal comprising either a rate modulated or a width modulated pulse stream. Signal conversion arrangements of this type are frequently used in connection with indirect d/a converters. If rate modulation is used, the output signal comprises one more pulses of a constant width with the number of pulses being dependent upon the binary value of the input signal. If width modulation is used, the output signal comprises a single pulse whose width is dependent upon the binary value of the input signal. Arrangements of this type are disclosed in section 7.4 on pages 204 et. seq. of Electronic Analog Digital Conversion by H. Schmid published by Van Nostrand Reinhold, New York in 1970.
Although both pulse rate and pulse width modulation are known and are suitable in many instances, both arrangements are less than ideal for certain applications. Pulse rate modulation is disadvantageous for input signals having a large number of data bits because of the high frequencies generated and because of semiconductor circuitry problems caused by signals having an excessively large number of transitions. For example, a sixteen bit wide input signal can have 2.times.2.sup.16 transitions (131072 transitions) for each word received. This number of transitions generates high frequency signals which can radiate throughout the circuitry of the utilization device for the rate modulated signal and cause problems if expensive precautions are not taken. The accuracy of a D-A converter with such a large number of transitions can be limited by timing accuracy of the transitions. For instance, if the pulses were 50 ns long, for a 16 bit D-A converter, a timing error in the transitions of 0.76 ps could cause a least significant bit error. Such a timing error could easily be caused by temperature changes, power supply variations, noise radiated from nearby circuits, etc. . Also, transitions of this magnitude can cause problems in the semiconductor circuitry used to generate the rate modulated signals. It is a characteristic of semiconductors that their junctions generate heat upon the reception of input signals with the amount of heat generated being dependent upon the number of transitions per second of the input signal. The large number of transitions resulting from input signals of a high binary value can generate sufficient heat so that the operating characteristics of the semiconductor devices are changed. This causes the devices to generate output pulses of differing energy values for different input signals. This is an intolerable situation for indirect digital to analog converters since their output pulses must each be of a uniform and predictable value. The reception of pulses having different energy values by output filters causes the analog output signal of the filters to vary by factors other than the digital value of the input signal. This represent an unacceptable situation for the high quality devices in which the D/A converter may be used.
Indirect d/a converters are known that use pulse width modulation. In these devices, the reception of each input signal generates a single output pulse whose width is determined by the binary value of the input signal. Generally, the larger the binary value of the input signal, the greater the width of the output signal that is generated. Although width modulation avoids the problems associated with rate modulation, width modulation has its own problems. The primary problem is the low frequencies involved for large value input signals. The worse case situation exists when the output signal is high fifty percent (50%) of the time and low fifty percent (50%) of the time. In such instances, the low frequencies involved require output filters that are large, costly, complex and slow in responding to changes in input compared to the relatively inexpensive filters that can be used for higher frequency signals. The use of large filters increases the size and/or the cost of the devices using the indirect d/a converters.
It may therefore be seen from the above that the known pulse rate and width modulation arrangements used in digital conversion circuits are disadvantageous and cause problems in the associated utilization devices.